Fluidized bed combustion (FBC) is a combustion technology used in power plants primarily to burn solid fuels. FBC power plants are more flexible than conventional plants in that they can be fired on coal, coal waste or biomass, among other fuels. The term FBC covers a range of fluidized bed processes, including circulating fluidized bed (CFB) boilers, bubbling fluidized bed (BFB) boilers and other variations thereof. In an FBC power plant, fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process, causing a tumbling action which results in turbulent mixing of gas and solids. The tumbling action provides a means for more effective chemical reactions and heat transfer.
During the combustion process of fuels which have a sulfur-containing constituent, e.g., coal, sulfur is oxidized to form primarily gaseous SO2. In particular, FBC reduces the amount of sulfur emitted in the form of SO2 by a desulfurization process. A suitable sorbent, such as limestone containing CaCO3, for example, is used to absorb SO2 from flue gas during the combustion process. In order to promote both combustion of the fuel and the capture of sulfur, FBC power plants operate at temperatures lower than conventional combustion plants. Specifically, FBC power plants typically operate in a range between about 850° C. and about 900° C. Since this allows coal to combust at cooler temperatures, NOx production during combustion is lower than in other coal combustion processes.
Boiler systems of FBC power plants are generally associated with limestone feed systems for sulfur capture. Processed limestone fed to a boiler is typically conditioned by means of size reduction machines to specific size ranges to allow for the desulfurization process to proceed efficiently.
Air systems in an FBC power plant are designed to perform many functions. For example, air is used to fluidize the bed solids consisting of fuel, fuel ash and sorbent, and to sufficiently mix the bed solids with air to promote combustion, heat transfer and control (reduction) of emissions (e.g., SO2, CO, CO2, NOx and N2O). In order to accomplish these functions, the air system is configured to inject air, designated primary air (PA), secondary air (SA), fluidizing air to a fluidized bed heat exchanger (FBHE), and over fired air (OFA), for example, at various locations and at specific velocities and quantities.
A distributed control system (DCS) is typically used to control the processes described above in an FBC plant, based upon operator input. To this end, an operator adjusts FBC system parameters in an attempt to maintain optimal operating conditions such as maximizing combustion of the fuel while minimizing SO2 emissions, for example.
In general, FBC power plants evolved from efforts to find a combustion process able to control pollutant emissions without external emission controls (such as scrubbers). Although FBC power plants have lower pollutant emissions than conventional combustion plants, ongoing efforts continually strive to reduce pollutant emissions to even lower levels. Thus, it has been found that combining an FCB plant, and more specifically, a CFB boiler, with an air pollution control (APC) system downstream from the CFB boiler provides increased sulfur capture over conventional power plants (even those utilizing conventional external emission controls such as scrubbers), as well as FBC power plants without the APC system. In addition to reducing overall sulfur capture at a back end of the combustion process, an APC system provides flexibility in operations of the FBC power plant. For example, by reducing SO2 emissions at the back end, less limestone is required upstream in the combustion process for a given SO2 emissions level.
Specifically, an APC system known as a flash dryer absorber (FDA) has been developed for use in conjunction with a CFB boiler to substantially enhance sulfur capture, thereby effectively reducing pollutant emissions to even lower levels. Thus, a CFB-FDA system offering a cost-effective solution for enhanced sulfur capture has been developed.
In a typical CFB-FDA system, a portion of sulfur is captured in the CFB boiler using in-furnace limestone injection, while additional sulfur is captured by the FDA in a backend process utilizing residual limestone in flying ash exiting the CFB boiler. Therefore, it is desirable to optimize the CFB-FDA as one integrated system, thereby allowing economically efficient use of limestone according to plant conditions at any given time, particularly during transients in plant operations.
In addition to an amount of limestone in an FDA, sulfur capture also depends upon temperature and relative humidity in the FDA. However, current FDA control systems neither measure nor control relative humidity in the FDA. Thus, it is desirable that an integrated CFB-FDA control system measure relative humidity and adjust operating parameters of the CFB-FDA accordingly.
Process and equipment integration and optimization of the CFB-FDA system is also needed. More specifically, CFB-FDA integrated processes are currently not controlled at economically optimum operating conditions. This is especially true during load changes and when other plant disturbances such as changes in ash loading, for example, occur. Complex relationships between many variables which affect performance of the CFB-FDA complicate efforts to control the integrated CFB-FDA process.
Conventional power plant simulators are limited to steam/water side process dynamics and only very simple combustion or furnace process dynamics are modeled; dynamic models of complex atmosphere control systems such as FDAs are not available at this time.
In addition, CFB boiler and APC controls are currently employed using separate hardware and software platforms for the CFB boiler and the APC. Thus, it is desired to develop a standardized package which can be integrated into an existing DCS of an FBC power plant using a CFB-FDA system. Developing a standardized control package will also enable implementation of the control package for use with FBC power plants which use different types of boilers, such as BFB boilers, as well as different APC systems such as selective catalytic reduction (SCR) and flue gas desulfurization (FGD) systems, for example.
Accordingly, it is desired to develop an integrated modeling and optimization control system for a fluidized bed combustion process and air pollution control system and, more specifically, an integrated modeling and optimization control system for a fluidized bed combustion process and air pollution control system which overcomes the shortcomings described above.